Stressed liquid crystals materials for light modulation

ABSTRACT

A new light modulating material using interconnected unidirectionally oriented microdomains of a liquid crystal, dispersed in a stressed polymer structure, is provided. The light modulating material is prepared by dissolving the liquid crystal in an uncured monomer and then curing the monomer so that the polymer forms a well-developed interpenetrating structure of polymer chains or sheets that is uniformly dispersed through the film. When the film is subjected to stress deformation the liquid crystal undergoes a change in its unidirectional orientation. The concentration of the polymer is high enough to hold the shear stress, but is as low as possible to provide the highest switch of the phase retardation when an electric field is applied. The new materials are optically transparent and provide phase modulation of the incident light opposed to the low driving voltage, linear electro-optical response, and absence of hysteresis. It has been shown that these new materials may be successfully used in display applications, optical modulator, and beam steering devices.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/299,993 filed Dec. 12, 2005, which is a continuation-in-part of U.S.patent application Ser. No. 10/365,322 filed Feb. 22, 2003, now U.S.Pat. No. 7,034,907 issued on Apr. 25, 2006 hereby incorporated byreference. U.S. Pat. No. 7,034,907 is incorporated herein by referencein its entirety. This application also claims the benefit of U.S.Provisional Application Ser. No. 60/635,428 filed Dec. 10, 2004, herebyincorporated by reference.

GOVERNMENT RIGHTS

The United States Government has a paid-up license in this invention andmay have the right in limited circumstances to require the patent ownerto license others on reasonable terms as provided for by the terms ofGrant No. 444226, awarded by the Defense Advanced Research ProjectsAgency (DARPA).

TECHNICAL FIELD

This invention relates generally to the technology of liquid crystaloptical elements and, more specifically, to the manufacture of newmaterial comprising a well developed stressed polymer structure thatunidirectionally orients microdomains of liquid crystals. The newmaterials are characterized by ease of formation and the capability toswitch large phase retardation within a short time by a low drivingvoltage.

BACKGROUND OF THE INVENTION

Light modulators operating at fast frame rates (kilohertz or faster) arein great demand for optical data processing and adaptive opticsapplications as well as for color projection displays using a timesequential color scheme. Much progress has been made in the last thirtyyears in developing optical switches or modulators, but current devicesare unsatisfactory for many applications. For instance, the majority ofactive fiber-optic devices used in present day systems, are based on anelectromechanical modulator. In one type, the optical fibers arepositioned end to end and mechanically moved in or out of line. Inanother type, mirrors are rotated to direct beams into or away from areceiving fiber. This can be accomplished mechanically or withpiezoelectric or electrostatic drivers. These mechanical devicesintrinsically lack speed and long term reliability.

Alternatively, fast (less than one microsecond) optical switches, usinga solid electro-optic crystal in which birefringence can be induced byapplication of an electric field to the crystal, have been developed.Operation is based on rotating the plane of polarization of light withrespect to the orientation of an analyzer that blocks or transmits lightdepending on the polarization direction. The basic arrangement worksefficiently if incoming light is polarized with a particularorientation. However, randomly polarized light suffers a loss. Thisdeficiency may be overcome by using additional elements that splitincoming light into two orthogonal polarizations, passively rotating oneto match the other, and combining the two into a single beam fed to thebasic modulator. However, the suggested electro-optic crystals requirevoltages of one kV or more for operation. Accordingly, such devices arenot well suited for many applications, including for telecommunicationdevices.

Additional modulators have been constructed using a tapered plate, aFaraday rotator or solid electro-optic crystal, and a second taperedplate. The Faraday rotator is controlled by varying the current in anexternal coil, which varies a magnetic field. But, the suggestedelectro-optic crystals require inefficient kilovolt drive voltages.Also, electrode design also effects polarization dependence andmodulation efficiency.

Liquid crystals are an interesting medium for electro-optical effectsdue to their large optical birefringence and dielectric anisotropy. Forexample, it is known to utilize a variety of modes of a liquid crystalcell such as π-cells, and optically controllable birefringent (OCB)cells. Unfortunately, such liquid crystal based light modulators haverelatively slow response times and cannot be operated typically fasterthan video rates (30-80 Hz). The transient nematic effect operating inthe reflective mode has been proposed to achieve fast response times ina liquid crystal cell. Fast speed is achieved by only utilizing thesurface layer of a nematic cell. The bulk of the cell remains unchanged.Utilizing only the surface produces only a low phase retardation.

To overcome the above limitations, liquid crystal devices containingpolymer have been developed over the past decades. These devices can bedivided in two subsystems: polymer dispersed liquid crystals (PDLC); andpolymer stabilized liquid crystals (PSLC). In a PDLC device, a liquidcrystal exists in the form of micro-sized droplets, which are dispersedin a polymer matrix. The concentration of the polymer is comparable tothat of the liquid crystal. The polymer forms a continuous medium whilethe liquid crystal droplets are isolated from one another. Thesematerials have been successfully used in displays, light shutters andswitchable windows. Further, there has been suggested an idea to usestretched PDLC films for producing electrically controlled polarizers.The operating principle of a PDLC polarizer is based on anisotropiclight scattering of PDLC films resulting from unidirectionally orientednematic droplets. The liquid crystal domains imbedded in the confinedgeometry of a polymer matrix are currently among the fastest knownswitching devices. Unfortunately, such systems have low filling factorsand liquid crystal domain size. Moreover, these devices are only knownto provide light amplitude modulation, but not light phase modulation,which is critical for beam steering applications.

To speed up the switching further, there have also been attempts tochange the shape of the droplets from the spherical to ellipsoidal. Thisidea was also realized to produce electrically controlled polarizers.The operating principle of a PDLC polarizer is based on anisotropiclight scattering of PDLC films resulting from unidirectionally orientednematic droplets. Unfortunately, such systems have low filling factorsand these devices are only known to provide light amplitude modulation,but not light phase modulation which is critical for variousapplications. Further, stretched PDLC devices, even at high shearingdeformations, never become fully transparent.

In a PSLC device, the polymer concentration is usually less than 10 wt%. The polymer network formed in such a liquid crystal cell is eitheranisotropic and mimics the structure of the liquid crystal or israndomly aligned. Because of the relatively low polymer content, thesize of the liquid crystal domains are relatively large (>λ), andtherefore, the switching times are not short enough to use in fastswitching devices. Higher polymer content produces more dense polymernetworks that result in significant light scattering in the cells.

In many cases, a desirable mode of operation is to switch large phaseretardation within a short period of time. The maximum retardation shiftΔL_(max)=(n_(e)-n_(o))d is a linear function of the cell thickness d,while the switching time varies as d². The n_(o) and n_(e) are theordinary and extraordinary refractive indices, respectively. When thefield is switched off, a typical liquid crystal cell withn_(e)-n_(o)≈0.2 and d=5 μm switches ΔL_(max)=1 μm withinτ_(off)=γ₁d²/π²K˜25 ms, where γ₁˜0.1 kg m⁻¹ s⁻¹ and K˜10⁻¹¹ N are thecharacteristic rotation viscosity and elastic constant, respectively.

Based upon the foregoing, it is evident that there is still a need inthe art for a liquid crystal device which has improved switching times,which can provide maximum phase retardation and still provide minimalscattering of light in the various modes.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a newliquid crystal light modulating material that decouples the thickness ofthe liquid crystal layer and the switching speed. The material comprisesinterconnected microdomains of a liquid crystal dispersed in a stressedpolymer structure. The stress deformation imposes unidirectionalorientation of the liquid crystal. The new material is opticallytransparent and provides electrically controllable phase modulation ofthe incident light.

It is another aspect of the present invention to provide a method ofmaking stressed liquid crystal film, comprising: mixing a solution ofliquid crystal material and a curable monomer; phase-separating thesolution to form a film with an interpenetrating structure of polymerand interconnected liquid crystal domains having their liquid crystaldirectors randomly oriented; and applying a force to the film to orientthe liquid crystal domains in a single direction and cause the film toappear substantially transparent. Application of an electric field tothe liquid crystal material reorients at least some of the liquidcrystal areas and generates a corresponding phase shift of lightimpinging the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

For a complete understanding of the objects, techniques and structure ofthe invention, reference should be made to the following detaileddescription and accompanying drawings, wherein:

FIGS. 1A-C are schematic drawings of a liquid crystal cell according tothe present invention: FIG. 1A is a schematic representation of the cellafter polymerization; FIG. 1B is a schematic representation of the cellafter application of a shearing force; and FIG. 1C is a schematicrepresentation of the cell after application of an electric field.

FIG. 2 shows scanning electron microscope (SEM) images of the films madeaccording to the present invention, wherein the scale is 10 μm.

FIG. 3 is a graphical representation of the transmittance spectra of SLCfilms prepared at different temperatures during the first step of thepolymerization.

FIG. 4 is a graphical representation of the transmittance spectra SLCfilms prepared at different temperatures during the first step of thepolymerization when the shearing deformation of 100 μm applied to thecells.

FIG. 5 shows how the transmittance spectra of the SLC film changes asthe applied deformation increases.

FIG. 6 is a graphical representation of transmittance vs. the wavelengthof light comparing the SLC cell according to the present invention toprior art of polymer dispersed liquid crystal cells.

FIG. 7 shows the dependence of the transmittance of the SLC cell vs.applied voltage measured between two crossed polarizers (λ=0.632 μm).The cell's thickness is 22 μm, the shearing distance is 50 μm.

FIG. 8 shows the dependence of the transmittance of the SLC cell vs.applied voltage measured between two crossed polarizers (λ=0.632 μm).The cell's thickness is 22 μm, the shearing distance is 80 μm.

FIG. 9 is another graphical representation of the dependence of thephase retardation of the SLC cell as a function of an applied electricfield.

FIG. 10 demonstrates the dependence of the phase retardation of the SLCcell as a function of an applied electric field when the voltageincreases (circles) and decreases (squares); the two curvessignificantly coincide showing absence of hysteresis.

FIG. 11 shows dynamics of the time ON for the SLC cell at theapplication of 100 V.

FIG. 12 shows dynamics of the time OFF for the SLC cell after removingof 100 V.

FIG. 13 is the diffraction pattern produced by the SLC cell: a, only thecentral spot of light can be seen without any voltage applied to thecell; b, application of a voltage to the cell led to appearance of adiffraction pattern on the screen. The most intense maximum of zeroorder is surrounded by two maxima of the first order; c, at some certainvoltages, disappearance of the diffraction maximum of zero order whileintensity of light in the diffraction maxima of the first order achievesits maximum value is observed.

FIG. 14 shows light intensity measured in the diffraction maxima of zeroand first orders as voltage ramps from 0 to 100 V. The wavelength of thetesting light is λ=1.55 μm.

FIG. 15 shows electro-optical performance of the SLC cell measured inthe zero order diffraction maximum: the graph on top-dynamics of thelight intensity change; the graph on bottom-dynamics of thecorresponding phase retardation produced by the cell.

FIG. 16 shows the dependence of the transmittance of the thick SLC cellvs. applied voltage measured between two crossed polarizers (λ=1.55 μm).The cell's thickness is 340 μm, the shearing distance is 450 μm.

FIG. 17 shows dynamics of the phase shift relaxation after removing 380V applied to the 340 μm thick SLC cell. The wavelength of the probinglight λ=1.55 μm; the cell switches phase retardation of 10 μm only 1 msand with the driving voltage of about 1 V/μm.

FIG. 18 shows: a) dependence of the transmitted light intensity vs.applied voltage for a 5 μm thick SLC cell with a low magnitude ofapplied shearing; b) dynamics of the total switching in OFF and ONstates when the driving voltage is 12 V.

FIG. 19 shows: a) dependence of the transmitted light intensity vs.applied voltage for a 5 μm thick SLC cell with a low magnitude ofapplied shearing; b) dynamics of the switching in OFF and ON states whenthe driving voltage is 4.7 V.

FIG. 20 shows: a) dependence of the transmitted light intensity vs.applied voltage for a 5 μm thick SLC cell with a high magnitude ofapplied shearing; b) dynamics of the switching in OFF and ON states whenthe driving voltage is 7.6 V.

FIG. 21 shows: a) dependence of the intensity vs. applied voltage for a5 μm thick planar cell filled with the pure 5CB liquid crystal; b)dynamics of the switching in OFF and ON states when the driving voltageis 1.2 V.

FIG. 22 shows dynamics of the switching in OFF and ON states for a 5 μmthick planar cell filled with the pure 5CB liquid crystal when thedriving voltage is 10 V.

FIG. 23 shows dynamics of the phase shift relaxation after removing 130Vapplied to the 25 μm thick SLC cell. With the wavelength of the probinglight λ=0.632 μm, the cell switches phase retardation of λ/2 within 40μs and therefore is able to provide light modulation with the frequencyof 25 kHz.

FIG. 24 shows the gradient of the phase retardation in the speciallydesigned SLC cell; 2D phase retardation distribution changes as afunction of an applied voltage.

FIG. 25 shows phase retardation distribution changes from 0.035 μm per 1mm of the cell's length at no applied voltage to zero when the appliedvoltage approaches 60 V.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and in particular to FIGS. 1A-C it can beseen that a liquid crystal cell according to the present invention isdesignated generally by the numeral 10. In FIG. 1A, the cell 10 includesa pair of opposed substrates 12 wherein at least one of the substrates12 is made from a transparent material such as glass or plastic andwherein the other substrate is either similarly transparent, black,colored, or reflective, such as provided by an aluminum substrate. Eachfacing surface of the substrates 12 has at least one electrode 14disposed thereon. Of course an aluminum substrate may itself function asthe electrode. As will be described in further detail, the electrodes 14may cover the entire surface of the substrate or the substrate may havea plurality of electrodes disposed thereon. For example, the electrodesmay be configured on one substrate in a plurality of rows which haveperiodic spacing therebetween. If desired, the other substrate may alsobe provided with a plurality of electrodes configured in columns so thatthe intersecting electrodes on the two substrates of the cell may form aplurality of pixels. It will be appreciated that the end use orapplication of this invention will likely dictate the configuration ofthe electrodes with respect to the substrates. In any event, thesubstrates are spaced apart from one another by a plurality of sphericalspacers 16 or equivalent structures, such as rods or other means knownin the art for maintaining a space between the substrates. If desired,but not required, alignment layers 18 may be disposed on the electrodesso as to assist in the alignment of the liquid crystal material to beused. Alternatively, other insulating layers may be applied to thealignment layers if desired. As will become apparent, no specializedsurface treatments that produce a preferred liquid crystal alignment arerequired to practice this invention. In other words, the electrodes mayhave direct contact with the material 20.

Filled in between the substrates 12 is a light modulating material 20which comprises a liquid crystal material and a monomer/polymermaterial. Preferably, the liquid crystal material is chosen from a groupthat includes nematic liquid crystals, cholesteric liquid crystals, andsmectic liquid crystals. In an embodiment, the light modulating material20, once filled between the substrates 12 is exposed to a predeterminedwavelength of ultraviolet light from a light source 34 so as topolymerize the monomer and to form liquid crystal domains or areas 22.Other processes to form such a structure within the material 20 arecontemplated, including such with thermosets and thermoplastics.

A voltage supply and appropriate control electronics system 30 isconnected between the electrodes 14 for applying an electric field tothe light modulating material 20. A switch 32 may be interposed betweenone of the electrodes and the power supply 30. As noted previously withrespect to an embodiment, a UV light source 34 may be utilized forpolymerizing the monomer so as to form the interpenetrating polymerchains 24 which extend between the surfaces of the substrates 12.

In a preferred embodiment, the material 20 is prepared in solution formand pre-separated with ultraviolet irradiation at an elevatedtemperature, such as above the nematic-isotropy transition temperatureof the liquid crystal material. Afterwards, the material 20 is cooled toroom temperature while still irradiating with ultraviolet light to forma film 26 of separate polymer 24 and liquid crystal domains 22 havingtheir liquid crystal directors randomly oriented.

It is known from the previous research of the PDLCs that the structureof the obtained polymer film depends on the temperature of thepolymerization. FIG. 2 shows the structure gradually changes fromball-like to polymer chains structure and then to the structure ofpolymer sheets when the temperature of polymerization changes from 42 to100° C. The most dramatic change in the structure occurs somewherebetween 60 and 70° C. that includes the nematic-isotropic (N−1)transition temperature of the liquid crystal that were used. The domainsof the liquid crystal become very small and the polymer chains appear tobe very tiny. Such structural changes suggested the procedure of thefilms polymerization in this particular example. First the cell wasirradiated at a uniform temperature higher than the N−1 transitiontemperature of the liquid crystal. Then the cell was irradiated at thetemperature of 20° C. (room temperature) with the UV light of the sameintensity. The first step created a very developed tiny polymerstructure while the second step finishes the phase separation moreeffectively and strengthens the structure created during the first step.For other examples, the desired polymer and liquid crystal domainstructure may be formed by other suitable techniques.

The structure of the polymerized film determines its transmittance. Thefilms, polymerized at a temperature lower than the N−1 transitiontemperature of the used liquid crystal, demonstrate higher scattering(FIG. 3; lower branch of the curves), while the films polymerized athigher temperatures are significantly more transparent (FIG. 3; higherbranch of the curves). The higher the temperature of the polymerization,the better is the transmittance. These data suggest that it may bedesirable to perform the polymerization step at a temperature that isabout 5-50° C., and more preferably about 10-30° C., above the N−1transition of the pure liquid crystal. For example, for the commerciallyavailable E7 liquid crystal, the best results were achieved when thepolymerization was performed ˜30° C. higher than the T_(N-I). To checkthe prediction further, SLC cells with the commercially available liquidcrystal E44 were prepared. This liquid crystal has a very close chemicalstructure to E7 but has higher values of Δn and Δε, and the N-Itransition temperature of the E44 is about 100° C. Thus, the films werepolymerized at a temperature in the region of 120-130° C. which producedthe necessary structure and optical quality.

It is desired to provide uniform orientation or alignment of the liquidcrystal directors throughout the thickness of the film or cell 10. Forexample, this can be accomplished by the application of a force, such asa mechanical shear, that orients all the liquid crystal molecules in thedirection of the applied force. The orientation is achieved throughoutthe thickness of the cell. In one embodiment it has been found that byholding one of the substrates 12 in a fixed position and applying adisplacement or shearing force 36 to the other substrate in a lineardirection provides the necessary application of force. The amount ofshearing has been found to correlate to the amount of phase shift forlight impinging upon the cell in the manner that will be discussedhereafter in further detail. Alternatively, the liquid crystal moleculescan be oriented or aligned by stretching the film 10 in a lineardirection. In other words, both ends of a film 10 could be grasped atopposite ends and pulled an appropriate amount by forces indicated bythe numeral 38. It is envisioned that other applications of mechanicalforce to either the cell 10 or the film 26 that is formed between thesubstrates will result in the desired alignment properties.

As best seen in FIG. 1C, application of an electric field causes theliquid crystal material to align in a homeotropic texture. In contrastto traditional PDLC films, the cell 10, which may also be referred to asStressed Liquid Crystals (SLCs), has vastly improved transmittanceproperties after shearing. SLCs scatter the light slightly after apreparation of the cell. Accordingly, application of an electric fielddoes not change the optical appearance of the SLCs film, but changes thephase retardation of the film wherein the liquid crystal molecules tendto orient along the electric field. Although not visible to the nakedeye, the changes in the orientation can be seen if the cell is placedbetween crossed polarizers. As shown in FIG. 1C, application of anelectric field by closure of the switch 32 or by use of the electronicssystem 30 drives the liquid crystal directors into the homeotropictexture, providing the change of the phase retardation. Of course, thefinal optical appearance of the cell depends on the polarization of theincoming light and the configuration of any polarizers on one or bothsides of the cell 10.

To produce oriented droplets of liquid crystal in the polymer matrix,the substrates were sheared relative to each other (FIG. 1C). Theobservation of the cell between crossed polarizers shows that this cellpossesses anisotropy in the direction of the shearing the substrates orof the force applied.

It was discovered that the intrinsic scattering of such films decreaseddrastically with the application of the shearing deformations. FIG. 4shows that this is valid for the films made at any temperature ofpolymerization. The region of transparency is broader for the films madeat a higher temperature. For example, the SLC film made at 100° C. hastransmittance of about 98% starting from 750 nm, whereas the SLC filmmade at 90° C. has transmittance of about 98% starting from 1250 nm.FIG. 5 shows how the scattering disappears with increase of the appliedshearing.

FIG. 6 shows difference in transmittance for the SLC material and thePDLC material made of the same components and kept under the sameshearing stress. The transmittance of the SLC cell does not depend onthe polarization while the transmittance of the PDLC cell does and ismuch lower for all polarization than the SLC cell's transmittance.

Because the liquid crystal domains were oriented unidirectionally, thestandard consideration for a uniaxial crystal can be used. When a planewave is incident normally to a uniaxial liquid crystal layer sandwichedbetween two polarizers, the outgoing beam will experience a phaseretardation δ due to the different propagation velocities of theextraordinary and ordinary rays inside the film, δ=2 πd(n_(e)-n_(o))/λ,where d is the cell gap, Δn is the birefringence and λ is thewavelength. For a homogeneous cell, the effective phase retardationdepends on the wavelength and the applied voltage. When the voltageexceeds the Freedericksz threshold voltage, the liquid crystal directoris tending to be oriented along the direction of the applied electricfield. As a result, the effective birefringence and, in turn, the phaseretardation is decreased. Thus one can electrically control the phaseretardation of the film. The process is reversible upon removal of thevoltage.

The electro-optic characteristics of the cells were measured by astandard method in the art (see, for example, “Electro-optic effects inliquid crystal materials” by Blinov and Chigrinov, Springer-Verlag, NY,1994). These methods are integrated in the Electro-Optic Measurements(EOM) software package developed at the Liquid Crystal Institute, KentState University. The cells were placed between crossed polarizers. Theoptical axis of the cells was set at 45° to the polarization directionof the polarizers. An electric field was applied to the electrodes ofthe cells and the dependence of the shift of the phase retardationproduced by the film on the applied voltage, V, was measured. Inaddition, the dynamics of the phase retardation shift after abruptswitching ON and switching OFF of the electric field was measured.

To demonstrate typical electro-optical behavior of the SLC material, a22 μm thick film using the liquid crystal 5CB and UV curable monomerNOA65 with the weight concentration of the components 90% and 10% wasprepared, respectively. The cell made of two ITO-glass substrates wasfilled with the mixture and irradiated with UV light in two steps: 30minutes with the UV light of the intensity of ˜30 m W/cm² at thetemperature of 60° C. and then another 30 minutes with the UV light ofthe same intensity and at the temperature of 25° C. After irradiation,the cell was placed in a specially constructed shearing device where theshearing distance was controlled with the accuracy of 5 μm.

FIG. 7 shows the dependence of the phase retardation as a function of anapplied voltage measured in transmittance mode for the SLC film when theshearing distance is 50 μm. The variation of the transmitted lightintensity between two successive minima demonstrates the switch of thephase retardation equal to the wavelength of the probing light, λ=0.632μm, or δ=2π in terms of the angular phase retardation. FIG. 7 shows thatto produce the phase shift of 3λ, ˜1.9 μm the SLC cell may require about68 V; 150 V applied to the SLC cell switches 2.2 μm of the phaseretardation. FIG. 8 shows the dependence of the phase retardation as afunction of an applied voltage for the same SLC cell when the shearingdistance is 80 μm. Counting the total number of maxima that can beproduced by the cell under application of different shearingdeformations, it was concluded that the higher shearing creates moreuniaxial liquid crystal alignment and the cell produced higher shift ofthe phase retardation. The expected maximum shift of the phaseretardation is Δnd(1−c)=3.78 μm, where Δn=0.191 birefringence of the 5CBliquid crystal, d=22 μm thickness of the cells, c=0.1 concentration ofthe polymer. If it is determined that the efficiency of the phaseseparation as a ratio of the expected maximum shift of the phaseretardation to those values obtained experimentally, it can be statedthat for this particular case the efficiency of the phase separation isabout 58%. This value may be significantly higher depending on thematerials used and film's preparation conditions.

FIG. 9 shows another representation of the phase retardation as afunction of an applied voltage. As one can see, the cell can be drivenin a linear regime when the induced shift of the phase retardationchanges linearly with the applied voltage. This particular cell canswitch about 1.6 μm of the phase retardation linearly with applicationof about 40 V. Such a behavior is not intrinsic to a cell filled with apure liquid crystal and may be due to a strong confining geometry inwhich liquid crystal is placed. Increase of shearing enlarges thelinearity region. It is demonstrated below in the specific examples thatsuch a feature can greatly simplify the driving schemes in manypractical devices.

The shearing eliminates the hysteresis that is intrinsic to all othersliquid crystal-polymer dispersions, including PDLC. FIG. 10 shows thedependence of the phase retardation of the SLC cell as a function of anapplied electric field when the voltage increases (circles) and when thevoltage decreases (squares); the two curves significantly coincideshowing absence of hysteresis.

Application of the shearing deformations to a SLC cell's substrateselongates the polymer chains, changes the shape of the liquid crystaldomains, causing them to have more prolonged shape as schematicallyshown in the FIG. 1C. Due of strong anchoring of the liquid crystalmolecules with the polymer material, the molecules of the liquid crystalaround of a polymer chain become oriented along the chain. All thesefactors together lead to the fast switching times. FIG. 11 shows that toswitch ON 3.5λ=2.2 μm of the phase retardation the SLC cell requiresabout 180 μs. The dynamics of the relaxation of the SLC cell afterremoving 100V is shown in FIG. 12. The relaxation time is about 2 ms.

It is also noted that the SLC film preparation technique according tothe invention is well developed and simple. Further, the active area ofthe film may be relatively large, the film does not require any liquidcrystal orientation layers that can reduce transmission through thecell, and the material operates well in a large range of temperatures.

Many modifications and variations of the invention will be apparent tothose skilled in the art in light of the foregoing detailed disclosure.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention can be practiced otherwise than as specificallyshown and described. The best modes of the invention are furtherillustrated and described by the following specific examples.

Example 1 SLCs for Diffraction Grating and Beam Steering Devices

Using the SLC material, an OPA device was built that allows steering aMWIR laser beam over 1 milliradian in 1 millisecond. For this purpose avariable retarder was created that is capable of producing 4.5 micronsof phase retardation in 1 ms. The structure and characteristics of theSLC OPA cell are described below in more detail.

The 5CB/NOA65 (90%/10%) mixture was sandwiched between ITO coated glasssubstrates and polymerized. The ITO layer on one glass substrate wasetched to give a series of parallel electrodes. The ITO layer on theother substrate was continuous. The planar orientation of the liquidcrystal was imposed by the shearing deformations in the directionperpendicular to the ITO stripes. The obtained SLC film was 22 μm thick.With zero voltage applied to the cell, all polarizations of light thatpass through the cell see a uniform refractive index and no diffractionoccurs. When a voltage is applied across the cell, those areas of thesuspension above the ITO electrodes switch from planar to homeotropic,while the other areas remain unchanged. This produces a periodicvariation in the refractive indexes and a phase grating, producingdiffraction of light that passes through the cell.

To visualize operation of the SLC cell, a visible light at thewavelength λ=0.632 μm was employed. A voltage was applied to every otherelectrode of the cell, keeping grounded all the other electrodes, andproduced a diffraction grating in the cell. The beam of a He—Ne laserpassed through a polarizer, the SLC cell, telescopic system, and adiaphragm and was registered by a photodiode. In such a scheme, thediffraction pattern produced by the SLC cell was extended by thetelescopic system to achieve a comfortable measurement o an opticalsignal in each of the diffraction maxima. The changes of the intensityof light were measured for the zero and first diffraction orders asvoltage ramps from 0 to 100 V. It was determined that light can be“pumped out” from the zero order maximum to the maxima of higher orders.The remaining intensity of light in the zero order was less than 1% fromits maximum value. About 83% of light transferred to the first ordermaxima as shown in FIG. 13.

The next two pictures show operation of the SLC cell registered at thewavelength of 1.55 μm. FIG. 14 shows change of the light intensity inthe diffraction maxima as voltage ramps from 0 to 100 V. The cellproduces a shift of the phase retardation of 2.25 microns measured intransmittance. FIG. 15 shows the dynamics of the cell relaxation asvoltage drops from 100V to 0V. All the changes of the phase retardationoccurred within the time of 2 ms.

Example 2 SLCs for a Large Shift of the Phase Retardation

A 340 μm thick SLC film was prepared using the mixture of the liquidcrystal 5CB and UV curable monomer NOA65 with the weight concentrationof the components 90% and 10%, respectively. The cell made of twoITO-glass substrates was filled with the mixture and irradiated with UVlight in two steps: 2 hours with the UV light of the intensity of ˜30mW/cm² at the temperature of 60° C. and then another 2 hours with the UVlight of the same intensity and at the temperature of 25° C. After theirradiation, the cell was placed in a shearing device where the shearingof 450 μm was applied.

FIG. 16 shows the dependence of the transmittance of the cell versusapplied voltage measured between two crossed polarizers (λ=1.55 μm). Thevoltage of 380V applied to this 340 μm thick cell creates the phaseretardation shift of almost 20 μm. FIG. 17 shows the dynamics of thephase shift relaxation after removing the 380V applied to the cell. Thecell switches phase retardation of 10 μm of total 20 within only 1 msand with the driving voltage of about 1 V/μm.

Example 3 SLCs for Display Applications

FIG. 21 A shows the dependence of the intensity versus the appliedvoltage for a 5 μm thick planar cell filled with the pure 5CB liquidcrystal material. FIG. 21B shows the dynamics of switching for a planar5 μm thick cell filled with pure 5CB liquid crystal that operates in theECB mode. The switching times between the dark and transparent statesare tens of milliseconds.

To compare, in this embodiment, thin SLC cells (˜5 μm) were produced fordisplay applications. The total phase shift that the cells are capableto provide is about 0.6 μm. FIGS. 19A and 20A show the dependence of theintensity of light that passes through the SLC cell placed between twocrossed polarizers vs. applied voltage for the case with low and highshearing applied to the cell, respectively. The driving voltage dependson the magnitude of the shearing deformations and varies between 4.7 forthe low shearing to 7.6 V for the high shearing deformation. Shearingalso determines the switching time. FIG. 19B shows dynamics of theswitching in OFF and ON states for the cell with lower shearing; time ONis about 2 ms and time OFF is about 4 ms. FIG. 20B shows dynamics of theswitching in OFF and ON states for the cell with higher shearing; timeON and time OFF are both about 2 ms. In both cases the switching of theSLC cell is accomplished within the timeframe that is order of magnitudeless than for the cell with pure liquid crystal.

Even large difference in switching times are observed when both cellsare changed between a planar and a homeotropic orientation. Atapproximately the same driving voltage the ON time for the SLC cell istwo times shorter while the OFF time is shorter by more than 50 times(compare FIG. 18.a and FIG. 22).

Example 4 SLCs for Ultra Fast Light Modulators

A 25 μm thick film was prepared using the liquid crystal E7 and UVcurable monomer NOA65 with the weight concentration of the components80% and 20%, respectively. The cell made of two ITO-glass substrates wasfilled with the mixture and irradiated with UV light in two steps: 30minutes with the UV light of the intensity of ˜30 mW/cm² at thetemperature of 100° C. and then another 30 minutes with the UV light ofthe same intensity and at the temperature of 25° C. After irradiation,the cell was placed in a shearing device where the shearing of 150 μmwas applied.

FIG. 23 shows the dynamics of the phase shift relaxation after removing130V applied to the SLC cell. The wavelength of the probing lightλ=0.632 μm; the cell switches phase retardation of λ/2 within 40 μs whenthe electric field is removed (relaxation). The switching ON time wasalso measured and it was realized that it might be 10 times shorter thanthe switching OFF time. Therefore this particular cell is able toprovide light modulation with the frequency of 25 kHz.

Example 5 SLC Films with Controllable Gradient of the Phase Retardation

Performance of a liquid crystal film is determined by an initial phaseretardation pattern created in the film and a way in which the patternchanges during application of an electric field. Here the fast switchingproperties of the stressed liquid crystals were combined with a gradientof the liquid crystal concentration in the plane of the SLC film.

Gradient of the liquid crystal concentration was imposed during thepreparation by the UV irradiation through the mask. The profile of thedifference of the phase retardation value in different areas isdetermined by the optical density profile of the mask and may be variedin a different manner in accordance with a particular application:centrosymmetric, cylindrical, saw-tooth profiled, etc. Those places withhigher liquid crystal concentration provide higher value of the phaseretardation. Conversely, the areas with a lower liquid crystalconcentration would have lower phase retardation. In addition, thedifference in the UV intensity may produce the domains of different sizein different areas of the cell. All these factors together lead to adifferent phase retardation shift when a uniform electric field isapplied over the entire area of the sample due to different amounts ofthe liquid crystal in different places of the sample are reoriented.

To demonstrate the described approach, a stressed liquid crystal filmwas fabricated consisting of E7 and NOA65 in a weight ratio of 86:14 andexposing it with UV light through a linear optical density filter.First, the phase separation was performed by polymerizing the film withUV light at the temperature of 110° C. for a half of hour followed by aroom temperature post cure for another 30 min. After polymerizationmechanical shear was applied.

The polymer network structures at different positions of the cell werecharacterized by scanning electron microscopy. It was observed that theregion exposed to stronger UV intensity has a higher polymerconcentration than the regions exposed to lower intensity. Also, themorphology of the polymer network changes a polymer-ball-like structurein the high intensity regions to thin polymer-sheet-like structuresexposed to lower intensity.

FIG. 24 shows 2D phase retardation distribution as a function of anapplied voltage. This distribution changes from 0.035 μm per 1 mm of thecell's length at no applied voltage to zero when the applied voltageapproaches 60 V (FIG. 25). Shearing not only reduced light scattering ofthe cell, sped up the response time, but it also enhanced the phaseretardation gradient. By adjusting a single applied voltage it waspossible to electronically control the optics of the refractive indexprism without complicated electrode patterns or electronics.

If an inhomogeneous centrosymmetric mask for the UV irradiation wasused, a switchable lens would be obtained. The focal length is relatedto the lens radius r, wavelength 2, and phase difference λδ as:F=πr²/λΔδ. In our experiment, r=18 mm, λ=0.632 μm, and Δδ=2π. Thus thecalculated effective focal length is around 180 m at no applied voltageand may be switched to infinity with the applied 60 V.

Depending on the liquid crystal gradient, a liquid crystal lens withfocus movable off as well as along the axis or switchable beam deflectormay be realized. In addition, these new prismatic SLCs can be used tomake electronically controlled tunable prisms and gratings, microlensarrays, and also other possible phase modulators simply by designingvariable patterned photomasks. The resulting devices can be addressedusing a single electrode and single applied voltage. This approach ismuch simpler than using complicated electrode patterns and complex driveschemes.

Liquid crystal based beam steering devices that use a continuousgradient in the index of refraction can be used to steer light to anangle α defined by sin α=Δn d/w, where Δn is the maximum value of thelinear change in the index of refraction along the aperture of width wand material thickness d. Generally the index of refraction referred tois the effective extraordinary index of refraction defined as

${n_{e}^{eff} = \sqrt{\frac{n_{e}^{2}n_{o}^{2}}{{n_{o}^{2}\sin^{2}q} + {n_{e}^{2}\cos^{2}q}}}},$

where q is the angle between the director and the light propagationdirection. It is considered that the steered beam of light is polarizedso as only to excite the extraordinary mode. The gradient in the valueof the index of refraction is typically created by using patternedelectrodes to create a gradient in the electric field strength along theaperture that causes a gradient in the orientation of the liquid crystaldirector, and the resulting value of the extraordinary index ofrefraction as related to the equation above. A problem is that as theaperture becomes large, a large steering angle α requires a largematerial thickness d. A common solution to this problem (that can beapplied if monochromatic light of wavelength λ is used), is to reset theΔn value when Δnd/λ, is an integer. In a liquid crystal device this canbe implemented by providing resets in the gradient of the electric fieldthat is created by the electrode structure of the device. However thissolution has another problem, in that it is difficult to create abruptchanges in the electric field strength along the aperture. Having apatterned electrode structure on the surfaces of the material ofthickness d can create a desired abrupt change in the voltage applied tothe material, but the abrupt change in potential is not maintainedthrough the thickness of the material due to “fringing fields”.Therefore the “fringing fields” prevent an abrupt change in the index ofrefraction needed to provide resets in the gradient of the index ofrefraction so that a large aperture, large angle beam steering devicecan be realized. The method of providing a linear change in the index ofrefraction described in example 4 does not require a patterned electrodestructure and does not suffer from the problems of “fringing fields”.The abrupt change in the index of refraction for resets is provided bythe polymer network that is constructed through the use of highlycollimated light that has a much lower degree of spreading that theelectric field strength of the patterned electrode approach.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

1. A method for preparing a liquid crystal light modulating cell, themethod comprising the steps of: mixing a liquid crystal material and acurable monomer to form a pre-polymerizing solution; providing a pair ofspaced apart substrates to receive the pre-polymerizing solution;disposing the pre-polymerizing solution between the spaced apartsubstrates; phase-separating the pre-polymerizing solution withultraviolet radiation at a temperature greater than a nematic-isotropictransition temperature of the liquid crystal material to form a polymerstructure of interpenetrating polymer chains which extend between thesurfaces of the substrates in the pre-polymerizing solution; cooling thepre-polymerizing solution to a temperature below the nematic-isotropictransition temperature while continuing the phase-separation of thepre-polymerizing solution by continued irradiation by ultravioletradiation to form strengthened polymer domains interspersed with theliquid crystal material, wherein directors of the liquid crystalmaterial are randomly oriented; and applying a force to the film todeform the polymer structure and orient the liquid crystal domains in adirection thereby adjusting the electro-optical properties of the liquidcrystal material including making the film appear substantiallytransparent.
 2. The method of claim 1, wherein each of the substrateshas at least one electrode disposed thereon.
 3. The method of claim 2,further comprising: connecting a power supply to each of the electrodessuch that application of an electric field by the power supply adjuststhe orientation of at least some of the liquid crystal directors andcauses a corresponding phase shift of any light impinging upon the film.4. The method of claim 3, further comprising: controlling an amount offorce applied to the film to control the amount of the phase shift. 5.The method of claim 2, wherein at least one of the spaced apartsubstrates is transparent.
 6. The method of claim 5, wherein one of thepair of spaced apart substrates is opposed by a reflective substrate. 7.The method of claim 5, wherein both of the substrates are transparent.8. The method of claim 3, wherein at least one of the substrates has aplurality of parallel electrodes.
 9. The method of claim 4, wherein alinear dependence of the phase shift versus applied electric field isobtained by adjusting the amount of force applied to the film.
 10. Themethod of claim 4, wherein hysteresis of the film is controlled byadjusting the amount of force applied to the film.
 11. The method ofclaim 3, further comprising: preparing the film with a gradient of thephase shift in which the magnitude of the gradient and its spacedistribution is determined by an optical density filter.
 12. The methodof claim 1, wherein substantially all of the liquid crystal areas areoriented along the direction of force applied to the light modulatingmaterial.
 13. The method of claim 1, wherein the force is applied byshearing at least one of the substrates relative to the other substrate.14. The method of claim 1, wherein the force is applied by stretchingthe film in a linear direction.
 15. The method of claim 1, wherein theliquid crystal material is selected from the group consisting of nematicliquid crystals, cholesteric liquid crystals, and smectic liquidcrystals.
 16. The method of claim 1, wherein the phase-separating stepis conducted at a temperature about 5 to 50° C. above thenematic-isotropic transition temperature of the liquid crystal material.17. The method of claim 1, wherein the phase-separating step isconducted at a temperature about 10-30° C. above the nematic-isotropictransition temperature of the liquid crystal material.
 18. The method ofclaim 13, wherein the shearing distance is greater than or equal toabout 50 μm.
 19. The method of claim 13, wherein the shearing distanceis greater than or equal to about 80 μm.
 20. The method of claim 1,wherein the ultraviolet radiation is passed through a mask resulting inthe formation of a plurality of network polymer domains at positionsthroughout the liquid crystal material.
 21. The method of claim 20,wherein the mask is an optical density filter.
 22. The method of claim20, wherein the mask has a shape selected from the group consisting ofcentrosymmetric, cylindrical, and saw-tooth profiled.
 23. The method ofclaim 20, wherein the polymer concentration and the shape of the polymernetwork domains are dependent upon the amount of ultraviolet radiationpassed through the mask and exposed to the pre-polymerizing solution.24. The method of claim 20, wherein the polymer network domains exposedto a stronger intensity of ultraviolet radiation result in a higherpolymer concentration.
 25. The method of claim 20, wherein the polymernetwork domains exposed to a lower intensity of ultraviolet radiationresult in a lower polymer concentration.
 26. A method of preparing aliquid crystal light modulating cell, the method comprising the stepsof: providing a pair of separated substrates, at least one electrodedisposed on a surface of each substrate and facing one another, and alight modulating material disposed between the substrates, wherein thelight modulating material includes a liquid crystal material and atleast one polymerizable material that is mixed to form apre-polymerizing solution; phase separating the pre-polymerizingsolution by exposing the pre-polymerizing solution to an intensity ofultraviolet radiation at a predetermined temperature, wherein theultraviolet radiation is passed through a mask resulting in theformation of a plurality of polymer network domains at positionsthroughout the liquid crystal material; applying a force to a film ofthe phase-separated pre-polymerizing solution to deform the polymernetwork domains and orient the liquid crystal material resulting in theadjustment of the electro-optical properties of the liquid crystalmaterial including its optical transparency; connecting a power supplyto each of the electrodes such that application of an electric field bythe power supply adjusts the orientation of at least some of the liquidcrystal director and causes a corresponding phase shift of any lightimpinging upon the film.